1. Field of the Invention
The present invention relates to a differential amplifying apparatus and, more particularly, to correction of the offset voltage of a differential amplifying apparatus.
2. Description of the Related Art
A delta-sigma (ΔΣ) A/D converter is known as a type of analog to digital (A/D) converters. When a signal having a direct-current (DC) component is input to a low-order ΔΣ A/D converter with a simple configuration, a signal having periodicity is superimposed on the digital signal output from the ΔΣ A/D converter. This periodic signal serves as “tonal noise” with a frequency changing depending on the magnitude of the DC component, and remarkably degrades the signal-to-noise (S/N) ratio.
To suppress the “tonal noise”, the DC component of the signal to be input to the ΔΣ A/D converter is made sufficiently small to lower the frequency of the “tonal noise” below the frequency band to be used. When using the output signal of a differential amplifier as the analog input signal to the ΔΣ A/D converter, the offset voltage output from the differential amplifier needs to be suppressed.
To suppress the output offset voltage of a differential amplifier, a technique disclosed in, for example, Japanese Patent Laid-Open No. 2006-311350 (literature 1) is usable. The offset corrector described in the literature 1 corrects the output offset voltage of a differential amplifier based on the voltage output from the differential amplifier when the inverting terminal and the non-inverting terminal of the differential amplifier are short-circuited by a switch without any feedback from the output to the input. That is, during the offset voltage correction period, the differential amplifier is operated while short-circuiting its two input terminals, and the offset is corrected based on the output voltage at that time. This operation enables to reduce the output offset voltage of the differential amplifier.
However, during the normal operation period after the offset correction period, the two input terminals of the differential amplifier are disconnected by the switch. The differential amplifier amplifies the difference signal between the input signals of the two input terminals and outputs the amplified signal. That is, the offset voltage between the input signals of the two input terminals cannot be corrected during the normal operation period. In other words, during the normal operation period, the offset voltage between the input signals of the two input terminals is amplified and therefore cannot be sufficiently corrected.